The board may on its own motion reopen or reconsider any case in which it has rendered a decision. Reopening or reconsideration of any case in which a decision has been made by the board, whether requested by the Attorney General or any other duly authorized officer of the Attorney General's office, or by the party affected by the decision, shall be only upon written motion to the board. Motions to reopen in deportation proceedings shall not be granted unless it appears to the board that evidence sought to be offered is material and was not available and could not have been discovered or presented at the formal hearing; nor shall any motion to reopen for the purpose of affording the alien an opportunity to apply for any form of discretionary relief be granted if it appears that the alien's right to apply for such relief was fully explained to him or her and an opportunity to apply therefor was afforded him or her at the formal hearing unless relief is sought on the basis of circumstances which has arisen subsequent to the hearing. A motion to reopen or a motion to reconsider shall not be made by or in behalf of a person who is a subject of deportation proceedings subsequent to his departure from American Samoa. Any departure from American Samoa of a person who is the subject of deportation proceedings occurring after the making of the motion to reopen or a motion to reconsider shall constitute a withdrawal of such motion.